1. Field of the Invention
The present invention relates to a resistance element suitable for use in for example a heating element, high temperature thermistor (temperature sensor), etc. and a method of production of the same.
2. Description of the Related Art
In the past, natural gas, propane gas, kerosine, and other gaseous fuels and liquid fuels have generally been ignited by using electrifying type resistance elements made of ceramics. This type of resistance element is required to have a superior thermal shock resistance and oxidation resistance in order to withstand the rapid rise in temperature, that is, reaching a temperature of over 1000xc2x0 C. in 2 to 3 seconds, and the high temperature of about 1550xc2x0 C. in the air. Further, even a resistance element for a high temperature thermistor for instantaneously measuring a high temperature of over 1000xc2x0 C. is required to have a superior thermal shock resistance and oxidation resistance in the same manner as such a resistance element for ignition.
To meet with this requirement, as the ceramic of the ceramic resistance element of the related art, use is made of silicon nitride due to its small thermal expansion coefficient and high strength.
A silicon nitride ceramic, however, is difficult to sinter, so a rare-earth oxide had been added as a sintering aid at the time of firing. This rare-earth has a detrimental effect on the silica film for preventing oxidation of the silicon nitride ceramic, so there was insufficient oxidation resistance of the resistance element at over 1400xc2x0 C.
In the case of a resistance element used at a steady state of at least 1000xc2x0 C., for example a resistance element used as a heater, the temperature of the resistance element has to be controlled by a computer etc. or else will rise excessively to about 30 to 100xc2x0 C. higher than the desirable usage temperature and the element will oxidize extremely easily.
Further, in a ceramic resistance element, it is necessary to embed an internal conductor for use as the heating resistor in the ceramic. Such an internal conductor has to be fired together with the silicon nitride ceramic, so has to have a melting point of at least 1800xc2x0 C. As such a conductor, tungsten, tungsten carbide, molybdenum, and chrome may be mentioned.
The thermal expansion coefficient of a silicon nitride ceramic, however, is 3xc3x9710xe2x88x926/xc2x0 C. at 0 to 100xc2x0 C., while the thermal expansion coefficients of the tungsten, tungsten carbide, molybdenum, and chrome of the internal conductor used as the resistor are all higher than that of the silicon nitride ceramic and therefore do not match it.
Therefore, a method of adding an insulating ingredient having a small thermal expansion coefficient such as silicon nitride or boronitride into the internal conductor for the purpose of reducing the difference in the thermal expansion coefficients (Japanese Unexamined Patent Publication (Kokai) No. 1995-239123) has been proposed.
If the amount of the insulating ingredient added in the internal conductor is increased to reduce the difference of the thermal expansion coefficients a bit, however, there is the problem that the resistance of the internal conductor increases and its properties deteriorate.
Further, when using tungsten or tungsten carbide as the internal conductor, part of the tungsten is silicified due to the firing process. As a result, the amount of the insulating ingredient in the internal conductor increases and there is the same sort of problem as the case of increasing the amount of addition of the insulating ingredient.
An object of the present invention is to provide a resistance element which can be used over a long time under a high temperature environment, has little fluctuation in resistance even with repeated rises and falls between room temperature and a high temperature, can withstand oxidation at a high temperature, and is otherwise superior in durability, and a method of production of the same.
The present inventors engaged in intensive studies to achieve this object and as a result discovered that by selecting a ceramic including xcex2-SIALON (Sixe2x80x94Alxe2x80x94Oxe2x80x94N) as the body of the resistance element or selecting a specific conductor material and insulator material as the internal conductor, it is possible to obtain a resistance element which can be used over a long time under a high temperature environment, has little fluctuation in resistance even with repeated rises and falls between room temperature and a high temperature, can withstand oxidation at a high temperature, and is otherwise superior in durability, and thereby completed the present invention.
That is, according to a first aspect of the present invention, there is a resistance element comprising at least
a resistance element body comprised of a ceramic including xcex2-SIALON of a composition expressed by Si6xe2x88x92zAlzOzN8xe2x88x92z (where, in the formula, z=0.3 to 1.0) and
an internal conductor embedded inside the resistance element body, wherein the internal conductor includes a conductor material containing tungsten and carbon and having an atomic ratio of carbon to tungsten of 0.4 to 1.1 and an insulator material and
the volume ratio of the insulator material to the conductor material is 0.25 to 1.5.
Preferably, the insulator material included in the internal conductor contains at least one material selected from silicon nitride, sillimanite, mullite, aluminum nitride, silicon oxynitride, and SIALON.
Preferably, the ceramic of the resistance element body contains less than 0.2 mol % of a rare-earth element oxide, provided that the mol % is calculated as an equivalent mol % of the rare-earth element alone.
Preferably, the rare-earth element oxide includes at least one of yttrium oxide, lanthanum oxide, and cerium oxide.
The method of production of the resistance element of the present invention is not particularly limited, but the following method is preferably used.
According to a second aspect of the present invention, there is provided a method of production of a resistance element comprising the steps of:
forming in a predetermined pattern, on a surface of a first green sheet for forming a ceramic including xcex2-SIALON of a composition expressed by Si6xe2x88x92zAlzOzNxe2x88x928 (where, in the formula, z=0.3 to 1.0),
a conductor paste for forming an internal conductor having a conductor material containing tungsten and carbon and having an atomic ratio of carbon to tungsten of 0.4 to 1.1 and an insulator material and having a volume ratio of an insulator material to the conductor material of 0.25 to 1.5;
laminating second and third green sheets of the same composition as the first green sheet on the upper and lower surfaces of the first green sheet to obtain a laminate unit; and
firing the laminate unit to obtain a resistance element body.
Preferably, the laminate unit is fired in an inert gas atmosphere or a reducing atmosphere.
Preferably, the laminate unit is fired at a temperature of 1300 to 1800xc2x0 C.
Preferably, the method further comprises a step of bonding external terminal electrodes through a solder material to takeout electrode portions of the internal conductor exposed at the two end faces of the resistance element body.
Preferably, the solder material is a silver solder material containing an active metal.
Preferably the external terminal electrodes are bonded by vacuum baking.
Preferably, the vacuum baking is performed at a pressure of 1.1xc3x9710xe2x88x924 to 8xc3x9710xe2x88x922 Pa and a temperature of 800 to 980xc2x0 C.
The xcex2-SIALON of the composition expressed by Si6xe2x88x92zAlzOzN8xe2x88x92z (where, in the formula, z=0.3 to 1.0) of the resistance element body comprises silicon, aluminum, oxygen, and nitrogen. The silicon in the xcex2-SIALON has a function of preventing further progress in oxidation, because the silicon can be oxidized to form a protective film comprised of a silicon oxide (SiO2) film on the surface of the ceramic. Therefore, the oxidation resistance at over 1400xc2x0 C. is also sufficient. If there is an alkali earth or rare-earth in the ceramic, a pure SiO2 protective film is difficult to be formed and a sufficient performance as a protective film can no longer be exhibited.
In the present invention, since the xcex2-SIALON included in the ceramic of the resistance element body comprises silicon, aluminum, oxygen, and nitrogen, at the time of use of the resistance element at a high temperature, the nitrogen diffuses out of the element body as nitrogen gas, the oxygen is used as the oxygen ingredient at the time of formation of the SiO2 protective film, and therefore the formation of a pure SiO2 protective layer is not inhibited. Note that aluminum promotes the densification at the time of firing, the formation of the SiO2 protective film is not inhibited at the time of use of the resistance element at a high temperature, and the mechanical strength is improved.
Further, since the internal conductor includes the specific conductor material and insulator material in a specific volume ratio and they do not melt at the firing temperature of the xcex2-SIALON, co-firing of the ceramic and internal conductor becomes possible.
Further, the thermal expansion coefficient of the specific conductor material is close to the thermal expansion coefficient of the xcex2-SIALON. Further, the specific conductor material does not easily react with the xcex2-SIALON.
Due to the above, according to the present invention, it is possible to provide a resistance element which (1) can be used over a long time under a high temperature environment of at least 1000xc2x0 C., preferably at least 1400xc2x0 C., more preferably at least 1600xc2x0 C., (2) has little fluctuation in resistance even with repeated rises and falls between room temperature and a high temperature of at least 1000xc2x0 C., preferably 1400xc2x0 C., more preferably 1600xc2x0 C., (3) can withstand oxidation at a high temperature of at least 1000xc2x0 C., preferably 1400xc2x0 C., more preferably 1600xc2x0 C., and is otherwise superior in durability.
Preferably, since the insulator material included in the internal conductor comprises at least one material selected from silicon nitride, sillimanite, mullite, aluminum nitride, silicon oxynitride, and SIALON, not only does the difference in heat expansion between the internal conductor and the xcex2-SIALON of the resistance element body become much smaller, but also the affinity increases and adhesion of the internal conductor into the resistance element body is improved.
The resistance element according to the present invention can be used for example as a heating element (such as ignition device), high temperature thermistor (temperature sensor), etc.
The present disclosure relates to subject matter contained in Japanese Patent Application No. 2000-42642 (filed on February 21), the disclosure of which is expressly incorporated herein by reference in its entirety.